Metal nanoparticles play important roles in many different areas. For example, they can serve as a model system to experimentally probe the effects of quantum-confinement on electronic, magnetic, and other related properties. They have also been widely exploited for use in photography, catalysis, biological labeling, photonics, optoelectronics, information storage, surface-enhanced Raman scattering (SERS), and formulation of magnetic ferrofluids. The intrinsic properties of a metal nanoparticle are mainly determined by its size, shape, composition, crystallinity, and structure (solid versus hollow). In principle, any one of these parameters can be controlled to fine-tune the properties of this nanoparticle. For example, the plasmon resonance features of gold or silver nanorods have been shown to have a strong dependence on the aspect-ratios of these nanostructures. The sensitivity of surface-enhanced Raman scattering (SERS) has also been demonstrated to depend on the exact morphology of a silver nanoparticle.
Many metals can now be processed into monodisperse nanoparticles with controllable composition and structure, and sometimes can be produced in large quantities through solution-phase methods. Despite this, the challenge of synthetically controlling the shape of metal nanoparticles has been met with limited success. On the nanometer scale, metals (most of them are face-centered cubic (“fcc”)) tend to nucleate and grow into twinned and multiply twinned particles with their surfaces bounded by the lowest-energy {111} facets. Other morphologies with less stable facets have only been kinetically achieved by adding chemical capping reagents to the synthetic systems. For examples, triangular nanoplates of gold have been synthesized by reducing chloroauric acid with citric acid (rather than sodium citrate) and by adding sodium hydroxide solution toward the end of this reaction. Silver nanoprisms in large quantities have also been prepared through a photo-induced approach in which small silver nanospheres transform to nanoprisms with the help of citrate and a co-ligand such as bis(p-sulfonatophenyl) phenylphosphine dehydrate dipotassium.
When a metal nanostructure is processed into a hollow entity, its performance can be further improved due to its relatively lower density and higher surface area than its solid counterpart. For instance, hollow nanoshells made of palladium have been shown to be an effective, well recoverable catalyst for Suzuki coupling reactions, while the monodisperse solid palladium nanoparticles greatly lose their catalytic ability after a single use.
Hollow nanostructures made of metals can be fabricated by depositing a thin layer of metal (or its precursor) on an existing solid nanostructure (e.g., silica beads and polymeric latexes) followed with the calcinations or chemical etching to remove the templates. However, a procedure for manufacturing hollow nanostructures with smooth, nonporous surfaces, homogenous, highly crystalline walls and structural integrity is needed.
Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings.